Again, this is a common FE problem attributed to a casting defect.
I will venture to guess that there were thousands of FE’s out there with the standard AERA repair done to them. At one point (mid-seventies) it was advised and considered good practice to do the repair when the block was bare and many were done without any sign or symptom just to avoid comebacks. If you look carefully at a few FE engines on the deck you can often spot the repair tube that has been installed.
The problem (crack, causing
oil in the water) can also be caused by debris in the head bolt holes being crushed and forced when the bolts were re-installed after say, a valve job at Joe’s service station. If this is what you have the sealing of those specific bolt holes may fix it temporarily, or until you choose to tear down the engine...
The
oil passage is right next to the threaded hole, it can be clearly seen if you bore away the cylinder for a "wet" sleeve.
All FE head bolt holes are “blind”